In a variety of commercially significant operations, it would be desirable to employ an optical microscope to image a substrate beneath and between a dense array of strips (such as strips having width less than 0.7 microns). For example, during the microlithographic step of semiconductor product manufacturing, it is desirable to measure the width of one or more photoresist lines in an array of photoresist lines deposited on a substrate. Typically, such structures are "1:1 dense arrays" of lines on the substrate (i.e., structures in which the spacing between adjacent lines deposited on the substrate is substantially equal to the width of each line). Measurements taken on a dense array of photoresist lines track the microlithographic process more accurately than measurements of the widths of isolated lines.
If the lines in a dense array (such as a 1:1 dense array) have width less than about 0.7 microns, conventional optical microscopes receive little or no deflected light from the substrate in the spaces between the lines. Making accurate linewidth measurements once the light signal from trenches between the lines has disappeared is an extremely difficult task. For this reason, until the present invention, semiconductor manufacturers have effectively been forced to use scanning electron microscopes to measure linewidths in dense arrays below 0.7 microns.
An advantage of the invention is that it provides a convenient and economical way to modify an optical microscope, to enable that microscope to image dense linewidth features in accordance with the invention. One type of optical microscope that can be modified in accordance with the invention is known as a confocal scanning optical microscope (CSOM). A primary advantage of a CSOM, which images samples one point at a time through an array of pinholes, is that it has a shallower depth of field than most other optical microscopes. Thus, a CSOM is able to resolve both height and width information, and to image (independently) areas of a sample which are separated in height by a wavelength with reduced interference.
Examples of CSOMs are described in Kino et al. U.S. Pat. No. 4,927,254, issued May 22, 1990, Kino et al. U.S. Pat. No. 5,022,743, issued Jun. 11, 1991, and in the paper by G. S. Kino and T. R. Corle, entitled "Confocal Scanning Optical Microscopy," Physics Today, 42, pp. 55-62 (September 1989).
The latter paper describes a CSOM in which light from an arc lamp propagates through a spinning Nipkow disk (a perforated disk through which a large number of holes have been drilled or etched in a spiral pattern). Each illuminated hole of the Nipkow disk produces a spot on the sample to be imaged. Light reflected from the sample propagates back through the disk to an eyepiece or camera. Many points on the sample are simultaneously illuminated by light through the holes of the Nipkow disk, so that the system effectively functions as a large number of confocal microscopes in parallel. The sample is scanned as the disk spins and the spinning spiral hole pattern sweeps the illuminated point pattern across the sample. As the disk spins, the system generates a real-time confocal scanning image of the sample.